In a method for producing an optical film by a solution extrusion method, a method to produce a lamination type optical film employing an acrylic resin and a cellulose ester resin via co-extrusion is disclosed in Patent Document 1. Its object is to provide a protective film for a polarizing plate having low retardation, low optical distortion, minimized bright spot foreign matters, good dimensional stability at high humidity, minimized curling, an adhesion property to glass substrate or the like. However it has been found that deviation of distribution of acrylic resin and cellulose ester resin in a layer occurs, and undulation and partial light leakage occurs since a resin solution is spread on an endless belt in case of co-extrusion in a solution extrusion method. Further it is apt to generate deviation of distribution of the additives in each layer as well as to generate curling.
On the other side, unnecessary stress suffers to interface between laminated films when a resin with high cooling speed shrinks, since cooling speed after film forming are different from each other depending on the kinds of resins to laminate in a laminate film via co-extrusion in a melt extrusion method. Consequently, there are problems that curling or insufficient adhesion occurs in some places, or above described undulation or partial light leakage generates.
A lamination type polarizing plate protective film of an acrylic resin and a cellulose ester resin via the melt extrusion method is disclosed in Patent Document 2. This improves strength at high moisture and temperature condition by forming a film which is three-layer construction composed of a cellulose acetate butylate resin having high moisture vapor permeability is sandwiched by a resin having moisture vapor permeability (such as acrylic resin), via film forming method using no solvent. However this improves change against moisture, the problems above described undulation and partial light leakage are not dissolved since there are problems of flatness deterioration due to wrinkle generated in cooling or drying process caused by difference in expansion and shrinkage property due to temperature and moisture between acrylic resin and cellulose ester resin, and processing troubles in film producing time such as film break at stretching or slitter process.